In general, a fuel cell is an energy conversion device converting chemical energy of fuel directly into electric energy. Such fuel cells have been intensively studied and developed as next-generation energy sources since fuel cells have high energy efficiency and cause little environmental pollution, and thus are eco-friendly.
Among such fuel cells, polymer electrolyte membrane fuel cells (PEMFCs) have been spotlighted as portable, vehicle or household electric power sources. This is because PEMFCs are advantageous in that they have a low driving temperature, avoid a water leakage problem caused by the use of solid electrolytes and have a high response rate. Additionally, PEMFCs are high-output fuel cells with a high current density as compared to other types of fuel cells, have a simple structure, and show high start and response characteristics. Further, PEMFCs allow the use of methanol or natural gas other than hydrogen as fuel, and have excellent durability. Moreover, PEMFCs allow downsizing thereof by virtue of their high output density, and thus have been studied continuously to be applied as portable fuel cells.
A polymer electrolyte membrane fuel cell comprises two electrodes and a polymer membrane functioning as an electrolyte. In general, a polymer membrane having a sulfonic acid group (—SO3H) introduced thereto has been used as the polymer membrane. While conventional sulfonic acid group-containing polymer electrolyte membranes show high conductivity and impart excellent quality to fuel cells under sufficiently humidified conditions, they undergo a rapid drop in the proton conductivity as the moisture content contained therein decreases, and thus cause significant degradation in the quality of a fuel cell under low-humidity conditions. Therefore, fuel cells using such conventional polymer electrolyte membranes need strict water control and a complicated system so as to be used at a high temperature of 100° C. or higher.
To solve the above-mentioned problem, many attempts have been made to develop various kinds of organic/inorganic composite electrolyte membranes by adding inorganic proton conducting fillers having high proton conductivity and hygroscopic property to organic polymers. However, such attempts are not applicable under low-humidity conditions use yet.